How do I add a ID to and image feild?
What do you mean? For what purpose?
http://doc.silverstripe.org/doku.php?id=imagefield will tell you about how to create an image field. If, however, you mean on a model level (which you may consider 'database' level), then all fields have IDs. So:
$has_one = array('Photo' => 'Image');
Will link an image to the page you're on (calling it Photo). Photo's ID can then be called with $this->PhotoID from within the page it's used.
I mean an ID that I can style.
I will be adding lots of images over time and I want to give them a specific ID tage which will allow me to style with css.